Indium-containing wafer and method of its manufacture

ABSTRACT

An indium-containing wafer from which removal of mercury can be reliably performed and a method of manufacturing such a wafer are provided in order to make the mercury C-V method, allowing characteristics of a the indium-containing wafer to be measured with high precision and being a non-destructive test, viable. An indium-containing wafer relating to the present invention is characterized by having, formed on its episurface layer, an added-on mercury-removal layer directed to removing wafer-surface adherent mercury and composed of a compound semiconductor. In addition, a method of manufacturing an indium-containing wafer relating to the present invention is characterized in that after evaluating electrical characteristics of the wafer with, as an electrode, mercury adhered onto the surface of the mercury-removal layer, the superficially adhered mercury is eliminated by removing the mercury-removal layer.

TECHNICAL FIELD

The present invention relates to indium-containing wafers, and moreparticularly to indium-containing wafers the surface of which has amercury-removal layer for removing mercury adhered to the surface of themercury-removal layer on the wafer for evaluating its electricalcharacteristics non-destructively, and to methods of manufacturing suchwafers.

BACKGROUND ART

Indium-containing wafers are widely used for photodetectors such as pinphotodiodes and avalanche photodiodes, light-emitting elements such aslaser diodes, and electronic devices such as field-effect transistorsand bipolar transistors. In order to comprehend the semiconductorcharacteristics of such indium-containing wafers, evaluatingcharacteristics of the wafers in terms of density, thickness, andcrystallinity is crucial.

With regard to evaluating such characteristics, capacitance-voltagemeasurement (hereinafter referred to as “C-V measurement”) has beenemployed for evaluating carrier density in and thickness of wafers;however, the mainstream C-V measurement to date has been the SPA(semiconductor profile analyzer) method. Due to the fact that the SPAmethod is one in which the C-V measurement is carried out while thewafer is being etched through its surface with an electrolyte, which isa solution, as an electrode, problems with the method have been errorsin the carrier density measurement, associated with fluctuations in thearea of wafer contact with the electrolyte, and degradation in depthaccuracy owing to etching unevenness; in addition, with the method beingessentially a destructive test only spot checks can be made and thus,measuring the carrier density and thickness of those wafers that arethemselves offered as manufactured articles has not been possible.

In contrast, the mercury C-V method—in which the C-V measurement iscarried out by providing a mercury electrode on the episurface of thewafer to employ a mercury-metal-compound-semiconductor Schottkybarrier—is free from the problems of errors in carrier densitymeasurement and degradation in accuracy with the SPA method; moreover,there is no surface etching with the mercury C-V method, which meansthat with mercury that has been adhered to the wafers as an electrodehaving been removed, wafers whose carrier density and thickness havebeen measured can be offered as manufactured articles. Furthermore, anadvantage with the mercury C-V method is that, in addition to the C-Vmeasurement a current-voltage measurement (hereinafter referred to as an“I-V measurement”) can be made, making it possible to gain insight intocrystallinity. Nevertheless, hazards to the human body and globalenvironment are feared, in addition to the problem of degradation inwafer characteristics, lest mercury be extant on wafers followingmercury C-V measurement.

DISCLOSURE OF INVENTION

An object of the present invention is to solve the foregoing problemsand make available an indium-containing wafer from which removal ofmercury can be reliably performed, and to afford a method ofmanufacturing such a wafer, in order to make the mercury C-V method,which allows characteristics of a target indium-containing wafer to bemeasured with high precision and is a non-destructive test, viable.

In order to accomplish the foregoing objective, an indium-containingwafer having to do with the present invention is characterized in havingan added-on mercury-removal layer composed of a compound semiconductorformed on its episurface layer. In addition, for the mercury-removallayer, a chemical mercury-removal layer with which, being soluble in amercury-solvent removing solution, the mercury and the mercury-removallayer together are cleared away, may be utilized; a physicalmercury-removal layer with which, being soluble in a removing solutionin which mercury does not dissolve or in a removing solution in whichmercury dissolves with difficulty, mercury is cleared away by themercury-removal layer being removed altogether, may be utilized. This isbecause in either case mercury adhering to the mercury-removal layer canbe reliably removed due to dissolution of the mercury-removal layer intothe removing solution.

Furthermore, an indium-containing wafer having to do with the presentinvention may be characterized in having two or more laminae, and inthat the etching speed, in respect of a removing solution for theoutside mercury-removal lamina, of the inside mercury-removal lamina is1/10 or less that of the outermost mercury-removal lamina. This isbecause the removal of mercury is made still more reliable by themulti-stage mercury-removal procedure, and diversified removal layerscan be designed.

In addition, a method of manufacturing an indium-containing wafer havingto do with the present invention is characterized by providing amercury-removal layer on an episurface of the wafer, and afterevaluating electrical characteristics of the wafer, with mercury adheredonto the mercury-removal layer being part of an electrode, removing theadhered mercury by clearing away the mercury-removal layer. Also, themercury-removal layer may be furnished bi-or-more-laminarly.

Herein, the electrical characteristics evaluation may be an evaluationof either the carrier density or the thickness of the wafer by means ofa C-V measurement technique, or may be an evaluation of either the darkcurrent or the breakdown voltage of the wafer by means of an I-Vmeasurement technique.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram of the design of mercury-removal layers;

FIG. 2 is schematic views of a mercury electrode part for mercury C-Vmeasurement, wherein (a) is a schematic sectional view of the entireelectrode part, and (b) is a schematic view of the mercury electrodesurface;

FIG. 3 is a schematic sectional view illustrating an indium-containingwafer according to one embodiment of the present invention;

FIG. 4 is graphs plotting, for an indium-containing wafer according toone embodiment of the present invention, wherein (a) is results ofmercury C-V measurement on the wafer prior to providing it with amercury-removal layer, and (b) is carrier densities calculatedthereafter at respective depths;

FIG. 5 is graphs plotting, for an indium-containing wafer according toone embodiment of the present invention, wherein (a) is results ofmercury C-V measurement on the wafer after providing it with amercury-removal layer, and (b) is carrier densities calculatedthereafter at respective depths;

FIG. 6 is microphotographs respectively showing the episurface layer ofan indium-containing wafer (a) prior to and (b) subsequent to a mercuryremoval operation (etching);

FIG. 7 is a schematic sectional view illustrating an indium-containingwafer according to another embodiment of the present invention; and

FIG. 8 is a schematic sectional view illustrating an indium-containingwafer according to yet another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A “mercury-removal layer” In the present invention refers to a compoundsemiconductor formed on a wafer episurface layer with the object ofremoving mercury adhering to the surface of the mercury-removal layer.Mercury-removal layers as termed herein cover chemical mercury-removallayers and physical mercury-removal layers.

A chemical mercury-removal layer means a layer in which, being solublein a mercury-dissolving removing solution, the mercury-removal layer isdissolved away together with mercury; and although it is notparticularly limited the layer may be, to name examples, an In—As-basedcompound semiconductor such as InAlAs, InGaAs, or InGaAs(P), in caseswhere an acid such as nitric acid, sulfuric acid, or phosphoricacid+aqueous hydrogen peroxide is the removing solution. “InGaAs(P)”herein refers to a compound having an As/(As+P) mole ratio of 0.5 orgreater. For example, the As/(As+P) mole ratio of an InGaAs(P) layerlattice-matched with an InP (001) substrate and of 1.3 μmemission-wavelength design is 0.6 to 0.7, which falls into the foregoingrange.

In turn, a physical mercury-removal layer means a layer in which, beingsoluble in a removing solution that does not dissolve mercury (whichcovers—likewise hereinafter—removing solutions that do not readilydissolve mercury), mercury is cleared away by the mercury-removal layerbeing dissolved off altogether. Although it is not particularly limitedthe layer may be, to name examples, an In—P-based compound semiconductorsuch as InP or InGaP(As) in cases where the removing solution ishydrochloric acid or the like, or an Si-based compound semiconductorsuch as SiN, SiO₂, and SiON in cases where the removing solution is anacid such as hydrofluoric acid, hydrofluoric acid+ammonium fluoride, orhydrofluoric acid+aqueous hydrogen peroxide.

Herein, “InGaP(As)” refers to compounds having a P/(P+As) mole ratio ofgreater than 0.5. For example, the P/(P+As) mole ratio of an InGaP(As)layer lattice-matched with an InP(001) substrate and of 1.0 μmemission-wavelength design is 0.7 to 0.9, which falls into the foregoingrange.

The compounds listed above as examples of Si-based compoundsemiconductors are ordinarily taken to be insulators, but may beutilized for the mercury-removal layer of the present invention sinceeven with a layer of such a compound provided on the episurface layer ofan indium-containing wafer the mercury C-V measurement can be madewithout hindrance. From this perspective, in the present specificationthe above-listed Si-based compounds too are regarded as semiconductorshaving a large energy gap and treated as being included in Si-basedcompound semiconductors.

It should be noted that when InGaAs(P) or InGaP(As) are utilized incombination, the relative etching speed with respect to the removingsolution is preferably 10 times or greater. In order for this to be thecase, it is preferable that the difference in As/(As+P) mole ratios orP/(P+As) mole ratios be 0.1 or greater. In this case, it is desirablethat the mole ratios of In and Ga be adjusted according as the moleratios of As and P, to lessen lattice mismatch with respect to thesubstrate.

Although the mercury-removal layer that is formed on the episurfacelayer of the indium-containing wafer is not particularly restricted,preferably selected is one with which the removing solution that clearsaway the mercury-removal layer does not dissolve the episurface layer.This is in order to prevent loss of the episurface layer. Herein, theepisurface layer of the indium-containing wafer is in some casesIn—P-based, while in others, In—As-based. Table I indicates the ease ofsolubility of mercury, In—P-based compound semiconductors, In—As-basedcompound semiconductors, and Si-based compound semiconductors in variousremoving solutions. Specifically, “0” in Table I indicates that thecombination dissolves easily, whereas “X” indicates that the combinationdoes not dissolve easily (is insoluble or dissolves with difficulty).

TABLE I Removing solution HNO₃ H₂SO₄ H₃PO₄ + H₂O₂ HCl HF HF + NH₄F HF +H₂O₂ Mercury Hg O O O X X X X Mercury-removing layer In-P-based InP X XX O X X X InGaP (As) In-As-based InAlAs O O O X X X O InGaAs InGaAs (P)Si-based SiN X X X X O O O SiO₂ SiON O: readily soluble X: insoluble ordissolves with difficulty

“Ease of solubility” as termed herein means those compoundsemiconductors with which relative etching speed with respect to eachremoving solution is 10 times or greater under defined conditions. Thisis because in order to reliably remove mercury that has been adhered tothe surface of an indium-containing wafer, the mercury-removal layermust be removed exclusively of the other wafer layers, and therefore alayer whose relative etching speed is less than 10 would be difficult toutilize as a mercury-removal layer.

FIG. 1 sets forth conceptual diagrams of mercury-removal layer designs.Those with a suffixed letter “m” after their reference numeral representmercury-removal layers. Reference numeral 10 indicates a layer otherthan the episurface layer of the indium-containing wafer, referencenumeral 11 an In—P-based compound semiconductor, reference numeral 12 anIn—As-based compound semiconductor. Reference numeral 13 m then denotesan Si-based compound semiconductor mercury-removal layer.

In cases where the episurface layer is In—P-based (11 in FIG. 1( a)),compound semiconductors that can be selected for the mercury-removallayer are those for which a removing solution that does not dissolvethis episurface layer (which covers—likewise hereinafter—removingsolutions that do not easily dissolve the episurface layer) is utilized,for example: an In—As-based compound semiconductor (12 m in FIG. 1( b))that uses as the removing solution an acid such as nitric acid, sulfuricacid, or phosphoric acid+aqueous hydrogen peroxide, or aqueous solutionsthereof; and an Si-based compound semiconductor (13 m in FIG. 1( c))that uses as the removing solution an acid such as hydrofluoric acid,hydrofluoric acid+ammonium fluoride, or hydrofluoric acid+aqueoushydrogen peroxide, or aqueous solutions thereof. Herein, an In—As-basedcompound semiconductor amounts to a chemical mercury-removal layer, andan Si-based compound semiconductor to a physical mercury-removal layer.

In cases where the episurface layer is In—As-based (12 in FIG. 1( d)),compound semiconductors that can be selected for the mercury-removallayer are those for which a removing solution that does not dissolvethis episurface layer is utilized, for example: an In—P-based compoundsemiconductor (11 m FIG. 1(e)), with the removing solution beinghydrochloric or like acid, or an aqueous solution thereof; and anSi-based compound semiconductor (13 m in FIG. 1( f)), with the removingsolution being hydrofluoric or like acid, hydrofluoric+ammonium fluorideor like acid, or an aqueous solution thereof. Herein, both an In—P-basedcompound semiconductor and an Si-based compound semiconductor amount toa physical mercury-removal layer. Various types of mercury-removallayers can be selected depending on the type of episurface layer in theindium-containing wafer and the type of cleaning solution, as justdescribed.

A preferred aspect of the invention renders the mercury-removal layertwofold or more, with the layers selected being ones in which theetching speed, with respect to the removing solution for the outsidemercury-removal layer, of the one-inside mercury-removal layer is 1/10or less that of the etching speed of the outside mercury-removal layer.This aspect is preferable because it makes the removal of mercury morereliable, and serves to diversify the mercury-removal layers tofacilitate architecting flexible manufacturing processes according tocircumstances. Selection of layers with a relative etching speed of1/100 or lower is a more preferable aspect, from the perspective offacility in manufacturing procedural operations.

FIG. 1 also illustrates outside mercury-removal layers selected withrespect to the inside mercury-removal layer, in cases where themercury-removal layer is twofold. For example, when the episurface layerof the indium-containing wafer is In—P-based (11 in FIG. 1( a)), themercury-removal layer may for instance be an In—As-based (12 m FIG. 1(b)) or Si-based (13 m in FIG. 1( c)) compound semiconductor, as notedabove; but these may be made an inside layer (inside mercury-removallayer), and further an outer-layer mercury-removal layer (outsidemercury-removal layer) may be provided. In this case, the outsidemercury-removal layer is selected so that the etching speed of theinside mercury-removal layer with respect to the removing solution forthe outer layer is 1/10 or less relative to the outside mercury-removallayer. If an In—As-based compound semiconductor is to be an insidemercury-removal layer, an In—P-based compound semiconductor (11 m inFIG. 1( g)) whose removing solution is hydrochloric or like acid, oraqueous solutions thereof, or an Si-based compound semiconductor (13 min FIG. 1( h)) whose removing solution is hydrofluoric or like acid, oraqueous solutions thereof, can be selected as the outsidemercury-removal layer. If an Si-based compound semiconductor is to bethe inside mercury-removal layer, an In—P-based compound semiconductor(11 m in FIG. 1( i)) whose removing solution is hydrochloric or likeacid, or aqueous solutions thereof, or an In—As-based compoundsemiconductor (12 m in FIG. 1( j)) whose removing solution is nitric orlike acid, or aqueous solutions thereof, can be selected as the outsidemercury-removal layer.

Likewise, in cases where the episurface layer of the indium-containingwafer is In—As-based (12 in FIG. 1 (d)), the mercury-removal layer maybe, to name examples, In—P-based (11 m FIG. 1( e)) or Si-based (FIG.1(0, 13 m) compound semiconductors, as noted above. In cases where anIn—P-based compound semiconductor is to be an inside mercury-removallayer, an In—As compound semiconductor (12 m in FIG. 1( k)) whoseremoving solution is nitric or like acid, or aqueous solutions thereof,or a Si-based compound semiconductor (13 m in FIG. 1( l)) that whoseremoving solution is hydrofluoric or like acid, or aqueous solutionsthereof, can be selected as the outside mercury-removal layer.Meanwhile, in cases where an Si-based compound semiconductor is to be aninside mercury-removal layer, an In—P-based compound semiconductor (11 min FIG. 1( m)) whose removing agent is hydrochloric or like acid, oraqueous solutions thereof, or an In—As-based compound semiconductor (12m in FIG. 1( n)) whose removing solution is nitric or like acid, oraqueous solutions thereof, can be selected as the outsidemercury-removal layer.

It should be understood that when the mercury-removal layer is madethreefold or more, a further outside mercury-removal layer can be chosenalong the same lines as described above.

The method of furnishing a mercury removal layer on the episurface layerof an indium-containing wafer is not particular limited, and a varietyof deposition methods may be used, such as a CVD (chemical vapordeposition) method, a MOCVD (metal-organic chemical vapor deposition)method, and a MBE (molecular beam epitaxy) method.

For the electrical characteristics of an indium-containing wafer, a C-Vmeasurement or I-V measurement is carried out in the following manner,with, as an electrode, mercury that has been adhered to the surface ofthe mercury-removal layer. FIG. 2 sets forth schematic views or amercury electrode part for mercury C-V measurement. In the figure (a) isa schematic cross-sectional view of the entire electrode part, and (b)is a schematic topside view of its inner-electrode and outer-electrodesections. The C-V measurement and I-V measurement are carried out on astage 24 that is furnished with an inner mercury electrode 21, an outermercury electrode 22, and a vacuum line 23. The ratio of the surfaceareas of the inner electrode and the outer electrode is 1:100.

The mercury C-V measurement is performed as follows. Anindium-containing wafer 26 relevant to the present invention is broughtso that its mercury-removal layer 25 comes into contact with the surfaceof the stage 24 having the inner electrode 21 and the outer electrode22, which are filled with mercury and a vacuum is drawn using the vacuumline 23 to contact-adhere the mercury electrode surfaces and themercury-removal layer surface.

By measuring the capacitance (C-V measurement) while voltage is appliedto the indium-containing wafer, the carrier density N(w) of theindium-containing wafer at a depth w from the surface is evaluated usingthe following equation (1).N(w)=(−C ⁶ /q∈A ²)(dC/dV)⁻¹  (1)

Herein, C represents capacitance measured using a dc reverse biasvoltage; q, the electron charge; ∈, the dielectric constant; A, themeasurement area; and dC, the change in capacitance with change involtage dV. Then the depth wis found from the following equation (2).w=∈A/C  (2)

Also, by measuring dark current generated when voltage is applied to theindium-containing wafer (I-V measurement), crystalline status of thewafer crystal can be evaluated from the fact that the greater the darkcurrent is, the more the crystal defects.

After evaluating carrier density, thickness, crystalline status, and thelike of the indium-containing wafer as just described, mercury that isextant on the mercury-removal layer is cleared away together with themercury-removal layer using a removing solution, when theindium-containing wafer having the mercury-removal layer is withdrawnfrom the mercury electrode.

Where an In—P-based mercury-removal layer is to be removed, hydrochloricor like acid, or an aqueous solution thereof, is utilized. An example ofthe removing solution that may be given is an aqueous solution in which36 mass % hydrochloric add is appropriately diluted.

Where an In—As-based mercury-removal layer is to be removed, an acidsuch as nitric acid, sulfuric acid, phosphoric acid+hydrogen peroxide,citric acid, or tartaric acid, or an aqueous solution thereof, isutilized. Alternatively, a mixed solution of two or more of these may beemployed. Examples of the removing solution that may be given are: anaqueous solution of 70 mass % nitric acid; an aqueous solution of 36mass % hydrochloric acid; an aqueous solution of a mixture of 85 mass %phosphoric acid and 30 mass % aqueous hydrogen peroxide; and an aqueoussolution of a mixture of citric acid and 30 mass % aqueous hydrogenperoxide.

Where an Si-based mercury-removal layer is to be removed, an acid suchas hydrofluoric acid, hydrofluoric acid+ammonium fluoride, orhydrofluoric acid+hydrogen peroxide, or an aqueous solution thereof, isutilized. Examples of the removing solution that may be given are: anaqueous solution of 50 mass % hydrofluoric acid; a mixed aqueoussolution of 50 mass % hydrofluoric acid and ammonium fluoride; and amixed aqueous solution of 50 mass % hydrofluoric acid and 30 mass %aqueous hydrogen peroxide.

Removal of mercury-removal layers, although not particularly restricted,can be performed by, for example, methods such as immersion-rinsing witha corresponding removing solution, or by ultrasonic cleaning. Subsequentto the elimination of the mercury-removal layer using a removingsolution, removing solution that is extant is washed off with water. Incases where the mercury-removal layer is two-or-more-fold, the steps ofremoving the mercury-removal layer using a corresponding removingsolution and the water wash are repeated. An indium-containing waferhaving undergone a final water wash is dried for delivery as a finishedarticle. There are no particular restrictions on the drying method, andnitrogen gas blowing, vacuum drying, heat drying, or the like can beadopted as appropriate insofar as the electrical characteristics of theindium-containing wafer are not compromised.

EMBODIMENTS Embodiment 1

FIG. 3 illustrates one example of the present invention. Asdevice-forming layers, a InGaAs(P)/InP/InGaAs multi-laminar structurelayer 32, and InP layers 33 and 34 having two laminae differing incarrier density, and respective thicknesses of 0.5 μm and 1.2 μm, wereepitaxially grown on a 50-mm-diameter n-type InP substrate 31, and ontop of them InGaAs(P) [As/(As+P) mole ratio=0.7] 35, serving as anepisurface layer, was epitaxially grown to have a thickness of 0.1 μm;thus an avalanche photodiode (APD) structure as an example of asemiconductor device in the present invention was obtained. With APDsthe carrier density in and thickness of the InP bi-layer 33, and theuniformity thereof, are crucial in device operations.

Further, a 0.1 μm-thick InP epitaxial layer 301 m, serving as anon-device-forming, mercury-removal layer, was formed on the episurfacelayer. The formation of the InP epitaxial layer was carried out usinghydrogen gas as a carrier gas and introducing trimethylindium andphosphine source materials Into a MOVPE (metal-organic vapor phaseepitaxy) furnace at a furnace pressure of 25 Torr and at a depositiontemperature of 650° C. In the case where an InGaAs epitaxial layer wasformed, the source materials used were trimethylindium,trimethylgallium, arsine, and disilane. The deposition rate was 2 μm/hin either case.

With the n-type indium-containing wafer prior to and subsequent to theformation of the mercury-removal layer, the results of a C-V measurementperformed according to a mercury C-V method and the calculation resultsobtained therefrom for carrier densities N(w) at various depths of theindium-containing wafer are plotted in FIG. 4 and FIG. 5. In bothfigures (a) shows a C-V curve with the horizontal axis representingpotential difference V (units: V) between the inner and outer electrodesand the vertical axis representing electric capacity C (units: F), while(b) shows an N-w curve with the horizontal axis being depth w (units:μm) from the surface of the indium-containing wafer and the verticalaxis being the logarithm of carrier density N (units: cm⁻³). From acomparison between both figures it is evident that high-precisionresults were obtained as to the carrier density, thickness, and theuniformity thereof in the area corresponding to the InP bi-layer layer33, regardless of the presence or absence of the mercury-removal layer.

Herein, the C-V measurement was performed by applying the potentialdifference of the inner electrode with respect to the outer electrode sothat it changed from −0.5 V to −20.5 V in a stepwise manner. The stepwidths were 0.1 V from −0.5 V to −1.5 V, 0.4 V from −1.5 V to −5.5 V,and 1 V from −5.5 V to −20.5 V. The I-V measurement was performed byapplying the potential difference of the inner electrode with respect tothe outer electrode so that it changed from −0 V to −15 V in a stepwisemanner with a step width of 0.25 V. In both cases a superimposed acvoltage with a frequency of 1 MHz and an amplitude voltage of 15 mV wasapplied to the inner and outer electrodes.

Next, the using a solution of a weight ratio 1:1 of 36 mass %hydrochloric acid and water as the removing solution the InPmercury-removal layer was removed, and thereafter the samples werewater-washed and dried. The indium-containing wafer surface was observedwith a microscope before the removal of the mercury-removal layer andafter the removal of the mercury-removal layer. As shown in FIG. 6 (thelongitudinal-side length of the photograph being 1700 μm), particles ofmercury were observed before the removal, whereas no residual mercurywas observed after the removal.

Meanwhile, the indium-containing wafer surface was immersed for 30minutes in an equimolar solution mixture of 100 mass % high-puritynitric acid and water heated at 90° C. to 100° C. before the removal ofthe mercury-removal layer and after the removal of the mercury-removallayer to dissolve the mercury remaining on the indium-containing wafersurface into the solution, mixture and thereafter the solution mixtureunderwent ICP (inductively coupled plasma) mass spectrometry formercury. At the same time an indium-containing wafer that was notbrought into contact with mercury was subjected to a control test in asimilar manner. Herein, the ICP mass spectrometry method refers to amethod in which qualitative and quantitative analyses are carried out byintroducing into a mass spectroscope ionized atoms copiously generatedin an ICP light source. The results of the ICP mass spectrometry are setforth in Table II.

TABLE II Detected amount of mercury (ng) Before removal of After removalof mercury-removal layer mercury-removal layer Sample 1 18 <0.05(Detection limit) Sample 2 14.5 <0.05 (Detection limit) Sample 3 12<0.05 (Detection limit) Control <0.05 (Detection limit) test

As indicated in Table II, it was confirmed that mercury can becompletely removed by a mercury removal operation utilizing removal ofthe mercury-removal layer. The concentration of mercury in the foregoingsolution mixture provided for the ICP mass spectrometry also proved tobe considerably lower than 50 ng/l, the limit defined by the WaterPollution Control Law, meaning that the indium-containing wafer that hasbeen subjected to the mercury removal operation using the removal of themercury-removal layer does not cause any environmental burden associatedwith mercury.

It should be noted that in an etching rate confirmation test that wasimplemented separately, in which a sample with a 3 μm InP single layerand a sample with a 1 μm InGaAs(P) single layer were treated in theforegoing removing solution (the removing solution in which 36 mass %hydrochloric acid and water are blended at a weight ratio 1:1) for 1minute, the InP single layer was etched 2.2 μm whereas theInGaAs(P)single layer was etched 0.01 μm, confirming that the relativeetching speed was 220 times faster.

Embodiment 2

FIG. 7 illustrates another example of the present invention. AnInGaAs(P)/InP/InGaAs multi-layered structure layer 72, and InP layers 33and 34 having two laminae differing in carrier density, and respectivethicknesses of 0.5 μm and 1.2 μm were epitaxially grown on a50-mm-diameter n-type InP substrate 71. In the present example the 1.2μm-thick InP layer corresponds to the episurface layer. Further on topof it, a twofold mercury-removal layer, a 0.1 μm-thick InGaAs layer 701m and a 0.1 μm-thick InP layer 702 m, were epitaxially grown in sequencein the same fashion as in Embodiment 1.

The n-type indium-containing wafer prior to and subsequent to theformation of the mercury-removal layer was subjected to a C-Vmeasurement in a similar manner to that in Embodiment 1, and carrierdensities N(w) at various depths win the indium-containing wafer werecalculated. In this case too, high-precision results were obtainedregardless of the presence or absence of the mercury-removal layer.

Next, the outer mercury-removal layer InP was removed in a similarmanner to that in Embodiment 1 whereas the inner mercury-removal layerInGaAs was removed using a solution of a mass ratio of 1:2 of 70 mass %nitric acid and water, and thereafter the sample was water-rinsed anddried. The episurface layer of the indium-containing wafer was thenobserved with a microscope and underwent ICP mass spectrometry, whereinno residual mercury was recognized.

Embodiment 3

FIG. 8 illustrates yet another example of the present invention. AnInGaAs(P)/InP/InGaAs multi-layered structure layer 82, and InP layers 83and 84 having two laminae differing in carrier density, and respectivethicknesses of 0.5 μm and 1.2 μm were epitaxially grown on a50-mm-diameter n-type InP substrate 81, and on top of it a 0.1 μm-thickInGaAs(P) [As/(As+P) mole ratio=0.7] 85, serving as an episurface layer,was epitaxially grown while a 0.1 μm-thick InP epitaxial layer 801 m wasformed, which served as an inside mercury-removal layer. Further, on topof it, a 0.1 μm-thick InAlAs epitaxial layer 802 m was formed, whichserved as an outer mercury-removal layer. The source materials used forgrowing this layer were trimethylaluminum, trimethylindium, arsine, anddisilane.

The n-type indium-containing wafer prior to and subsequent to theformation of the mercury-removal layers was subjected to a C-Vmeasurement in a similar manner to that in Embodiment 1, and carrierdensities N(w) at various depths w in the indium-containing wafer werecalculated. In this case too, high-precision results were obtainedregardless of the presence or absence of the mercury-removal layer.

Next, the outer mercury-removal layer InAlAs was removed using a aqueoussolution mixture of a mass ratio of 1:2:20 of 70 mass % nitric acid, 30mass % aqueous hydrogen peroxide, and water, whereas the innermercury-removal layer InP was removed in a similar manner to that inEmbodiment 1, and thereafter the sample was water-washed and dried. Theepisurface layer of the indium-containing wafer was then observed with amicroscope and underwent ICP mass spectrometry, wherein no residualmercury was recognized.

It should be noted that the foregoing examples involve the handling ofmercury, and therefore safety and hygienic considerations are necessary.In particular, immediately after the C-V measurement mercury remains inthe wafer; for this reason, protective wear such as protective glovesand protective eye glasses, as well as implementation of localventilation are indispensable in order to prevent the mercury fromcoming into contact with the human body and from being inhaled.Additionally, in the removal of the mercury-removal layer, appropriatehandling for mercury is required since the removing solution containsmercury. Further, due to the possibility of mercury adhering to the jigsused, sufficient operational management and washing are crucial.

The embodiments and samples disclosed herein are in all aspectsillustrative and not to be construed as restrictive. The scope of thepresent invention is defined by not the foregoing description but theappended claims, which are intended to include all modifications andequivalents of the claims.

INDUSTRIAL APPLICABILITY

The present invention thus makes it possible to evaluate electricalcharacteristics of indium-containing wafers with high precision and in anon-destructive way using a mercury C-V method or the like regardless ofthe presence or absence of mercury-removal layers, and to remove mercurythat has adhered by removing the mercury-removal layer; thus, it makesavailable indium-containing wafers whose electrical characteristicsaccording to objectives are guaranteed.

1. A semiconductor device comprising: an indium-containing wafer;device-forming layers deposited on said indium-containing wafer, saidlayers defining an episurface device-forming layer; and a dissolvable,mercury-adhering non-device forming layer composed of InGaAs(P) fordissolving in a nitric acid, sulfuric acid, or phosphoric acid+aqueoushydrogen peroxide removing solution, said mercury-adhering layercomprising at least two laminae each of composition having an As/(As+P)mole ratio different from that of the immediately adjacent other lamina,such that the difference between the As/(As+P) mole ratio of one laminaand the As/(As+P) mole ratio of the immediately adjacent other lamina is0.1 or greater, and said mercury-adhering layer formed, as the outermostlayer on the semiconductor device, on said episurface device-forminglayer, for carrying a mercury electrode for profiling characteristics ofthe semiconductor device-forming layers.
 2. A semiconductor devicecomprising: an indium-containing wafer; device-forming layers depositedon said indium-containing wafer, said layers defining an episurfacedevice-forming layer; and a dissolvable, mercury-adhering, non-deviceforming layer composed of InGaP(As) for dissolving in a hydrochloricacid removing solution comprising at least two laminae each ofcomposition having a P/(P+As) mole ratio different from that of theimmediately adjacent other lamina, such that the difference between theP/(P+As) mole ratio of one lamina and the P/(P+As) mole ratio of theimmediately adjacent other lamina is 0.1 or greater, and saidmercury-adhering layer formed, as the outermost layer on thesemiconductor device, on said episurface device-forming layer, forcarrying a mercury electrode for profiling characteristics of thesemiconductor device-forming layers.
 3. A semiconductor devicecomprising: an indium-containing wafer; device-forming layers depositedon said indium-containing wafer, said layers defining an episurfacedevice-forming layer; and a dissolvable, mercury-adhering, non-deviceforming layer composed of at least two laminae, one of InGaAs(P) and animmediately adjacent other of InGaP(As), each of composition such thatthe difference between the As/(As+P) mole ratio of the InGaAs(P) laminaand the As/(As+P) mole ratio of the InGaP(As) lamina is 0.1 or greater,said mercury-adhering layer formed, as the outermost layer on thesemiconductor device, on said episurface device-forming layer, forcarrying a mercury electrode for profiling characteristics of thesemiconductor device-forming layers said mercury-adhering layer must beremoved from the semiconductor device in order to render operable thedevice formed by said device-forming layers.